For more than three decades, cable TV programmers enjoyed steady ratings gains—mostly at the expense of broadcasters. Inspired efforts like AMC’s Mad Men and FX’s Sons of Anarchy notwithstanding, those steady ratings increases sometimes belied the quality of programming they ran on their channels (sorry, Snooki). But with the cable business finally reaching saturation when it comes to adding new subscribers, and viewers blessed with more on-demand choices than ever, many top cable channels have recently experienced their worst ratings declines ever.

As the Wall Street Journal was first to notice Monday, ratings across top cable TV networks have taken a big, unexpected hit in the first quarter of this year. Measuring total day ratings performance, 15 of the top 20 basic cable channels experienced ratings drops in the first quarter—a rather unheard of benchmark in recent years, when channels steadily grew their audience as U.S. cable, satellite and telco TV service providers added subscribers. From 2001 to 2011, ad-supported cable’s full-year ratings average increased from a 26.6 rating to a 37.4, according to metrics researcher Nielsen.

But as paidContent reported last week, subscriber growth is down for multichannel operators, with the previously explosive expansion of telco services Verizon FiOS and AT&T U-Verse finally ebbing.

Perhaps unable now to tap a regular infusion of new cable and satellite subscribers, big cable channels like USA Network, TNT and—most notoriously—Nickelodeon are seeing major ratings declines, in some cases for the first time ever.

Measuring the first quarter through March 18, data released by Turner Networks showed that Viacom-owned Nickelodeon continued its conspicuous fourth-quarter losses with a 24 percent total viewership drop in the first part of this year. NBC Universal-owned USA Network, a perennial basic cable top dog and home to top-rated prime-time original series Burn Notice, declined 5 percent across all day parts; it also dropped 5 percent in total prime-time viewership and 9 percent in prime-time viewership among adults 18-49, the demographic its ads are sold on.

Turner-owned TNT, meanwhile, declined 17 percent in total viewership and 11 percent in total prime-time viewership. Among its money demo, adults 25-54, the network also dropped 12 percent in prime time. And Disney-owned ESPN saw its total viewership taper off 5 percent during the period—it dropped 3 percent for adults 18-34 in prime time, the combination the sports network cares most about.

Some of the loss can probably be attributed to cable-channel-to-cable-channel migration. For example, at TBS—Turner’s comedy-focused answer to TNT—prime time ratings were up a whopping 33 percent during the quarter to date. History Channel, meanwhile, posted prime-time viewership gains of 20 percent.

But 75 percent of the top 20 channels on cable TV losing viewership share in a quarter has never happened, so this will be something we keep our eye on as the year progresses.

Republished with permission from paidContent, which writes about the transformation of the media-and-entertainment industries in the digital era, with a focus on emerging-business models and technologies.